Pulse shaping circuits



Oct 20 1959 WAY noNG woo 2,909,659

PULSE SHAPING CIRCUITS Filed Sept. 5. 1957 United States Patent gO PULSESHAPING CIRCUITS Way Dong Woo, Newton Center, Mass., assignor toRaytheon Company, a corporation of Delaware Application sepfembers,19s7serialNo.6s1,7'26

6 claims. (ci. 25o-27) This invention pertains to pulse shaping circuitsand more particularly relates to an electronic circuit responsive to apulse or train of input pulses of lvaryingamplitudes for generating acorresponding pulseorl train of output pulses of uniform andpredetermined amplitude. The novel circuit is capable of generatingrectangular pulses having steep leading and trailing edges.

The invention makes use of the properties of certain magnetic materialscharacterized by a rectangular hysteresis loop providing twoidentifiable stable states of magnetization and providing a highlynonilinear ilux density-coercive force relationship whereby the state ofmagnetization can be definitely and quickly changed. The inventionutilizes a magnetic'core transformer having its secondary windingconnected in series with the load and having a constant DC. biasingcurrent applied to its primary winding to cause the magnetic core to beheld at one of its two stable states of magnetization. An input pulsecauses a rapidly increasing current to flow in the secondary windinguntil the magnitude of that current causes the magnetic core to commencechanging its state of magnetization. At this point a large change inmagnetic ux occurs in the core inducing aback in the secondary windingwhich tends to oppose any further increase of current and maintain-thesecondary winding current at a constant value. After vtermination of theinput pulse, the D.C. biasing current vin ythe primary winding causesthe magnetic core to return to its initial state of magnetization. Byvirtue of this action, the n ovel pulse shaping circuit is capableofproducing an output pulse having steep leading and trailing edges andhaving a predetermined amplitude. By adjustment of the D.C. biasingcurrent flowing in the primary Winding, the amplitude of the outputpulse may be changed. The ease and rapidity with which the amplitude ofthe output pulse may be changed is an important attribute of thisinvention.

The invention and its mode of operation can be apprehended by referenceto the following exposition when considered in conjunction with thedrawings wherein:

Fig. l is a schematic representation of the preferred embodiment of theinvention;

Fig. 2 is a diagram showing a rectangular hys-teresis loop; and

Fig. 3 is a schematic representation of another embodiment of theinvention. y

Referring now to Fig. 1, there is illustrated `a circuit for regulatingthe amplitude of a current pulse. A vacuum tube, here shown as a triode1, has its cathode connected to ground through a resistor 2 in serieswith the secondary winding 3 of a transformer 4 having a core fabricatedof magnetic material characterized by a rectangular hysteresis loop. Inthe primary winding 5 of the transformer, there is a large biasingcurrent ID@ supplied from a constant current DC. source connected toterminals 6'. A constant current generator may be any current generatorwhose internal impedance is high `in lrelation to the impedance of theload supplied. The dots adjacent the windings of transformer d, inaccordance With conveny 2,909,659 Patented Oct. 20, 1959 ICC-tion,.indicate that the primary and secondary windings are positionedaround the core so that the same electrical polarity obtains at thedotted ends of those windings. A diode 7 is shunted yacross 'the ends ofsecondary winding 3 to permit electron lflow to ground. The plate oftube 1 is connected to a source of B+ voltage through the load which isconnected across output terminals 8. The control grid of tube 1 isconnected through resistor 9 to input terminali() and a diode 11 isprovided to clamp the grid to ground when the input signal exceedsground potential.

lPig. 2 shows the type of rectangular hysteresis'loop whichcharacterizes the magnetic core of transformer 4. The magneticproperties of the core material are dened by the way the ux density B isrelated to the applied magnetic field H. VWhen the core` is magnetizedby an applied field, in the negative direction., for example, ofsuflicient magnitude to cause saturation, the core will retain thelarger part of its magnetization when the applied lield is removed. lfthe rectangularity were perfect the magnetic flux density -Br would vbeequal to the saturation flux density .-Bm and when once driven tosaturation there would be no further change in the state ofmagnetization duetto additional negative lields'of any magnitude.Certain materials such as magnesium man- -ganese ferrites, permalloy,nickel-iron alloys, and others exhibit a high degree of rectangularity.Because the magnetic core of transformer 4 ischaracterized by arectangular hysteresis 1oop,.the current IDCi in primary winding 5causes flux density saturation of the core and the initial operationalpoint is established at the point 12 by the magnitude of the directcurrent. H1 is the coercive force established by .current flow in theprimary winding, i.e.

where k is a constant characteristic of the .geometrical dimensions ofthe core, and N1 is the number of turns in the primary winding.

Inorder to cause the magnetic core toV change its state ofmagnetization, it is necessary to provide an opposing coercive force H2somewhat greater in magnitude than H1. When such an opposing coerciveyforce is applied, a large and rapid change influx occurs in the core.The opposing coercive force H2 is generated in the circuit of Fig. lbycausing current I2 to ilow in the proper direction in secondarywinding 3 and H2=kN2l2 Where N2 is the number of turns in the secondarywinding, and I2 is the current tlowing in the secondary winding.

In the circuit of Fig. 1, tube 1 is cut oif in the quiescent state byimpressing'a fnegative bias potential Ec., on the control grid. Becauseof the D.C. biasing current iiowing in primary Winding 5 the operatingpoint of the core of transformer 4 is at point 12 of the hysteresisdiagram. When a positive pulse `ec is applied at input terminal 1), thecontrol grid is driven to ground potential and kept at that potential byclamping diode 11. Current rapidly commences to flow through the tubesince the secondary winding 3 oifers but slight impedance due tosaturation of the magnetic core. As tube current flow increases theoperating point shifts to the right from point 12 and when `the rcurrentI2 in the secondary winding causes `the coercive force H2 to ,exceed H1,the operating point moves up the knee of |the hysteresis curve towardpoint 13. When this occurs a high voltage develops across the secondarywinding because of the rapid change in flux density in the magnetic coreand consequently the voltage of the cathode is raised. The grid-cathodebias is thereby reduced ,sufficiently to cause the output current I2 toremain almost constant. The .output current will now tend to increase.slowly but before the operating point reaches saturation in thepositive direction (-l-Bm) the grid driving pulse ecshould be completed.The pulse@c in the drawing is of such duration that the operating pointmoves to point 13 on the hysteresis diagram before the pulse isterminated. Upon termination of the input pulse the tube 1 is again cutoif and the collapsing iield about the windings causes the diode 7 toconduct whereby the cathode is rapidly returned to ground potential. Thebiasing current in primary winding 5 concurrently causes the operatingpoint to return to the initial point via path l13, 14, and 12 so thatthe circuit will be ready for another input pulse. Diode 7 is employedto prevent the cathode from dropping below ground level during the fallof I2. Because of the rapid collapse of the magnetic field, ringing maybe encountered in the cathode circuit and in this circumstance aresistor of appropriate value placed in .series with diode 7 will dampout the undesired oscillations. The pulse shaping circuit provides ahigh degree of degeneration during the peak of the output current, andextremely small degeneration during the rise and fall of the outputcurrent, giving fast rise and fall times. Resistor 2 in the cathodecircuit provides some degeneration and its function is to protect tube 1from excessive plate current in the event that the control grid shouldbe driven up for too long an interval.

The peak output current attained is JWM. H. I2 N2 LICN,

where Hc is the coercive force of the core.

Using a core of low coercive force this is essentially NrDo N2 Theoutput current can be varied by changing ID C which determines theinitial operating point 12. By decreasing Inca the initial operatingpoint is moved to the right and the amplitude of the output pulse isdecreased. As a corollary, increasing IDC, causes the initial operationpoint to be moved to the left and the amplitude of the output pulse isincreased. Since the output of a constant current generator may, ingeneral, be easily regulated, the initial operating point of the core isreadily changed to control the amplitude of the output pulse.

fFig. 3 illustrates a current amplitude regulatingcircuit which may beused where Ithe impedance of the load and the input signal source areboth very low. A biasing current ID C supplied by a constant currentgenerator connected to terminals 15, is caused to flow in the primaryWinding 16 of transformer 17. The core of the transformer is constitutedof a magnetic material of the type previously described having arectangular hysteresis loop. The secondary winding 21 of transformer 17is shunted by a rectiiier 25 in series with a resistor 26. YThe inputsignal pulse ec is applied at terminals 18 and a low impedance load,here indicated as a resistor l19, is connected across terminals 20whereby the secondary winding 21 is in series with the load. Thedirection of current liow to and from the load is indicated by `arrowslabeled IL. A rectifier 22, in series with secondary winding 21,prevents load current IL from reversing. Where a rectifier may alreadybe present in the load or the signal source, rectifier 22 may beeliminated.

The current Inc. iiows in the primary winding 16 in such direction as tocause the magnetic core of transformer 17 to besaturated in the plusdirection and the initial operating point to be established at the point23 indicated in Fig. 2. When the positive input pulse ec is applied atterminals `18, it causes a rapidly increasing current IL to flow insecondary winding 21 and when the coercive force due to the current flowin that Vwinding exceeds the coercive force due yto current IDC. flowingin the primary winding, the operating point of the core moves down theknee of the hysteresis curve toward point 24. At this time a high backis induced across the secondary winding because of the rapid change inux density in the core thereby opposing any additional increase of thecurrent IL. The current IL will now tend to increase slowly but beforethe operating point reaches saturation in the negative direction (-Bm)the input pulse ec is completed. The diode 25 will now conduct currentdue to the collapse of the magnetic eld about the transformer windingsand the resistor 26 acts to prevent ringing or oscillations in thesecondary winding 21. The diode 25 also acts as a safety device inasmuchas it pre- 'vents a high inverse voltage from being impressed across therectifier 22 upon collapse of the magnetic ield. The biasing current inprimary winding 16 causes the operating point of the core to return toits initial position to prepare the circuit for the next input pulse.

While there has been described what is at present considered to be thepreferred embodiment of this invention, it will be obvious to thoseskilled in the electronic circuit art that various changes andmodications may be made therein without departing from the scope of theinvention as set forth in the appended claims.

What is claimed is:

1. A pulse shaping circuit comprising a transformer including a primaryand a secondary winding and having a magnetic core characterized by arectangular hysteresis loop, means for impressing a potential on saidprimary Winding to cause a biasing direct current to iiow therein, aunilateral impedance and a load connected in series with said secondarywinding, and means for connecting an input signal source in series withsaid secondary winding.

2, A pulse shaping circuit comprising a primary and a secondary windingand a magnetic core, said magnetic core having a rectangular hysteresisloop, a direct current generator coupled to said primary winding wherebya biasing current flows in said primary winding causing flux densitysaturation in said core, a unilateral impedance and a load connected inseries with said secondary winding, and means for connecting a source ofinput signals in series with said secondary winding, said input signalscausing current to flow in said secondary winding in a direction suchthat the coercive force attributable to that current opposes thecoercive force due to said biasing current in said primary winding.

3. An electronic circuit comprising a transformer including a magneticcore having a rectangular hysteresis loop, a direct current generatorcoupled to the primary winding of said transformer, an electron tubehaving a cathode, anode, and control grid, said cathode being connectedthroughsaid secondary winding of said transformer to a referencepotential, means coupled to said control grid for applying inputsignals, unilateral impedance means connected to said control grid forclamping said grid to said reference potential when said input signalsexceed a predetermined potential, and means for connecting a load inseries with said secondary winding.

4. A pulse shaping circuit comprising a transformer including a magneticcore of the type having a rectangular hysteresis loop, a constantcurrent generator coupled to the primary winding of said transformer, avacuum tube including a cathode, an anode, and a control grid, saidcathode being connected through the secondary winding of saidtransformer to a reference potential, means connected to said controlgrid for clamping said grid to said reference potential, means forconnecting said anode to a potential source, and means for connecting aload in series with said anode.

5. A pulse shaping circuit comprising a transformer having a primary anda secondary winding positioned on a magnetic core of the typecharacterized by a rectangular hysteresis loop, a constant directcurrent generator coupled to theprimary winding of said transformer, avacuum tube including a cathode, anode, and control grid, said cathodebeing connected through sai-d secondary winding toa reference potential,a unilateral impedance means connected to said control grid for clampingsaid grid to said reference potential, a unilateral impedance meansshunted across said secondary winding, means for connecting .a load inseries with said secondary winding,

having a primary and a secondary winding positioned on a referencepotential, means coupled to said control grid 10 for impressing inputsignals, a unilateral impedance coupled to said control grid forclamping said grid to said reference potential when said input signalsexceed a predetermined potential, means for connecting a load in serieswith said secondary winding, a unilateral impedance shuntcd across saidsecondary winding, and means for coupling said anode to a plate voltagesource.

References Cited in the le of this patent UNITED STATES PATENTS2,688,075 Palmer Aug. 31, 1954 2,825,820 Sims Mar. 4, 1958 FOREIGNPATENTS 763,734 Great Britain Dec. 19, 1956 1,116,389 France Ian. 30,1956

